The CFPB says banks cannot charge an overdraft fee on one-time debit card transactions and ATM withdrawals unless you opt in to overdraft protection. They may charge overdraft fees on checks and recurring electronic payments even if you did not opt in.
Declining overdraft protection means no fee on debit and ATM transactions, but overdrawing with a check or recurring electronic payment can still cost a fee. Some banks let you opt out of those overdrafts, but you may then face a non-sufficient funds fee, often the same amount as an overdraft fee, and declined payments can trigger a returned-item fee from the merchant.
If you are enrolled, you can change your mind at any time by telling your bank or credit union. If you do not believe you authorized debit overdraft protection and were charged a debit overdraft fee, you can file a complaint with the CFPB.